The investigators have previously shown that the population vector of recorded motor cortical neurons predicts accurately and continuously the arm trajectory during a volitional movement. It is still unclear how this predictive signal relates to the muscle contractions used to move the arm and how it will interact with a perturbation in controlling arm movement. If an unexpected perturbation is applied to an animal s arm after its intended motion has already started, there will be deviations in movement trajectory and reactive changes in muscle activity. One would expect to see corresponding changes in the cortical activity patterns. This change may gradually evolve as the perturbation becomes a fixed feature of the movement. The investigators propose to examine this change in neuronal activities under both novel and adapted conditions toward the perturbation. The long term goal of this project is to understand control strategies used by the sensorimotor system as it interacts with an environment containing unexpected perturbations. The intent is that the information obtained through this investigation will help to develop control systems that utilize cortical signals to control neuromechanical prostheses or functional neuromuscular stimulation systems for humans with brain/spinal cord injury or other neurological damages. The proposed methodologies involve perturbing primate arm motions while simultaneously recording activities from a population of cortical neurons through a chronically implanted fine wire array electrode with up to 96 channels. Rhesus monkeys will be trained to perform a 3D, unrestrained, visually guided center->out reaching task. During each movement the investigators will simultaneously record and correlate arm trajectory, muscle activity and cortical cell activity. They will apply a transient perturbation to the arm by applying a sudden pulling force at the wrist after the initiation of the movement. This perturbation will be applied during every movement toward each of the eight pseudo-randomly presented targets. The effect of the perturbation on arm trajectory will be reduced as the animals learn that the perturbation is a fixed feature of the task. The perturbation will then be removed to examine the after-effects. They will examine the temporal relation among the activities of individual cortical cells and their population vectors, muscle activities, and endpoint movement directions and velocities, both before and after the perturbation. This is achieved by comparing data from four different phases of the experiment: the training phase for control data, the novel phase when the perturbation is first applied, the adaptation phase after the animals have learned the perturbation dynamics and effect, and the extinction phase upon removal of the perturbation. After the monkey adapts to the perturbations, they expect to see a predictive strategy demonstrated in the neuronal and muscle activity before the onset of the perturbation. The 3-D dynamic model of a monkey arm will be refined and used to evaluate strategies adopted by the monkey to minimize the effects of the perturbation.